It is well known that internal combustion engines of the movable piston type are inefficient converters from chemical energy to mechanical energy. A major cause for this inefficiency lies in the fact that the combustion of the fuel by the explosion is not complete in two respects. in the first place, there is generally a significant portion of the fuel which is not oxidized at all, and is simply swept out of the cylinder on the exhaust stroke and dumped into the air. In the second place, there is a further significant portion of the fuel which is oxidized only in part. Oxidation of carbon to carbon monoxide releases only one-third as much heat as oxidation of carbon to carbon dioxide. While carbon dioxide is a principal component of engine exhaust gas, it is well known that significant quantities of carbon monoxide are also present, to an extent requiring caution because of its toxic nature. The well known catalytic converter has as one of its functions the oxidation of carbon monoxide to carbon dioxide, a relatively innocuous substance. The conversion, of course, releases the remaining heat, but this now occurs where it cannot be availed of mechanically: it simply raises the temperature of the converter, and thus is a problem in heat disposition.
In the effort to obtain more complete combustion in the engine cylinders, the trend has been to make the mixture as lean as possible, that is, to supply as great a proportion of air as can be tolerated, so that oxygen will be available for combustion. It has also been found that the thermal efficiency of an engine varies with the compression ratio, other things being equal, so a second trend has been to raise the compression ratio as high as other limitations, such as temperature tolerances, will permit.
An unanticipated result of burning relatively lean fuel mixtures after high compression has been the appearance in the exhaust gases of oxides of nitrogen. These oxides are of even greater detriment to the environment than the carbon monoxide, and are not rendered innocuous by a catalytic converter.
One type of emission control device currently in use is the catalytic converter. A very recent type of catalytic converter, known as the "3-way" catalytic converter is described in the November 1978 issue of Popular Science at pages 64-72. This type of catalytic converter generates a considerable amount of heat, yet fails to use the heat for any power utilization. Another type of emission control system is more directly related to the design of the piston engine. This type of a system is the stratified charge system used by the Honda Motor Company. In the stratified charge system, the piston engine has two combustion chambers for each cylinder, a main chamber and a pre-chamber. On the intake stroke, a rich fuel-air mixture enters the pre-chamber through a separate intake valve, and a very lean mixture enters the main chamber in the conventional manner. Under the compression stroke, a moderate fuel-air mixture forms near the pre-chamber opening in the main chamber. The spark plug fires the rich mixture which in turn ignites both the moderate and lean charges.